Analysis of the contact electrification in metal-metal, metal-dielectric and dielectric-dielectric contacts applying broadband ultra-sensitive charge detection

• Applicants: Professor Dr. Rolf Möller; Professor Dr. Hermann Nienhaus
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518939313

Despite the fact that contact electrification and triboelectricity has been observed for more than 2000 years the underlying physics are still unclear. We have developed an experimental technique which enables us to study the charge transfer between a free-falling spherical particle and a surface with unprecedented precision. It relies on self-built charge amplifiers providing a resolution < 1 fC and < 1 µs. Experiments performed for the contact between metals reveal that the particles are discharged during the contact but regain within less than a microsecond unexpectedly high charge when the electric contact breaks. Depending on the impact velocity the charge is much higher than predicted by the generally accepted model by Harper and Lowell. Experiments including a ring electrode in proximity to the bottom plate demonstrate an even enhanced charge resolution. In addition, the extended setup allows to detect shifts between the center-of-mass and the center-of-charge of the sphere. The goal of this project is to analyse the key processes of contact electrification between metals (e.g. Au, Cu, Pt) and between metals and dielectrics (e.g. Si, InP, Al2O3, ZrO2) as well as between two dielectric materials. In the first period of the research program we accomplished a new experimental setup inside a vacuum chamber. It allows the in-situ preparation of the capacitor plate and the neutralization of the spheres before the free fall as well as the transfer of new samples into the setup without breaking vacuum. First experimental results on the electrification of Au and Cu spheres in contact with Cu(OH)2/Cu and hydrogen passivated Si(111) surfaces reveal that the surface potential and the electronic states at the contact area are the crucial factors for the amount and polarity of the transferred charge. The experimental scope will be extended to enhance and control the contact electrification between metals by use of thin dielectric adlayers, e.g., for contacts on TiO2/Ti or CuO/Cu. In cases of semiconducting and insulting materials we continue the successful experiments to investigate the role of adsorbate or defect-induced acceptors and donors at the interface in the contact electrification. For semiconductors we use Si and InP surfaces of both dopings, n- and p-type. The implementation of the ring electrode allows to determine any asymmetric charge distribution on an insulating sphere before and after contact. It is intended to prove that in case of insulating spheres the transferred charge remains at the spot of contact. Due to the neutralization feature in the new experimental setup this kind of sensitive experiments becomes feasible. Additionally, experimental evidence of flexoelectric polarization may be achieved by detecting signals which are related to the force applied by the sphere onto the plate during contact. The experiments are expected to deliver a more profound insight into the mechanisms of contact electrification.

• DFG Programme: Research Grants